1. Field of the Invention
The present invention relates to an electrophotographic type or electrostatic recording type image forming apparatus having at least five image forming units for forming multi-color images, such as a copying machine and printer.
2. Description of the Related Art
With progress of the image forming apparatus in recent years, the demand for image quality has been increased and thus, an electrophotographic type image forming apparatus which adds additional colors to a conventional four-color image forming apparatus has been proposed. In such an apparatus, color toners such as pale cyan and pale magenta (hereinafter referred to as light cyan and light magenta) are added to conventional general four colors, yellow, magenta, cyan and black (Japanese Patent Application Laid-Open No. 05-35038). These pale color toners are added to intensify the image quality by reducing the granularity.
In some cases, a color difficult to reproduce by mixing yellow, magenta and cyan is formed as a special color like a printer.
As a configuration of the image forming apparatus using toners of not less than the four colors (hereinafter referred to as multi-colors), apparatuses shown in FIGS. 9 and 10 have been known (Japanese Patent Application Laid-Open No. 2005-202027).
In the tandem type shown in FIG. 9, development units 115a to 115f loaded with developers having different spectroscopic characteristics, respectively, are arranged in series corresponding to six image bearing members 111a to 111f. In this type, the output speed of an image can be equalized even if considering in basis of the four-color image forming apparatus, thereby providing a configuration emphasizing productivity. This system utilizes an intermediate transfer belt S as its intermediate transfer member.
However, the above-described technology has the following problems.
In the case of forming six colors using light cyan and light magenta in the tandem type considering productivity, a space which the image forming unit and the light scanning device require is 1.5 times compared with the image forming apparatus for forming the four colors.
Even if the interval of the image forming unit is decreased by reducing the sizes of a photosensitive drum, development units, cleaners and the like and changing the shape thereof for miniaturization, the height of the optical scanning device needs to be increased or a number of reflection mirrors for reflecting a light beam needs to be used because the optical scanning device has a predetermined optical path length.
As a method for reducing the space occupied by the optical scanning device, a method of reducing the optical path length is available. However, as a disadvantage due to reduction of the optical path length, there occurs a problem that the curvature of field is increased. That is, because a difference between a moving amount of light at an end portion and a moving amount of light at a central portion in a main scanning direction increases, the spot diameter of laser beam is different between the central portion and the end portion. Thus, if the optical path length of laser beam to be irradiated to the image bearing members for forming toner images using yellow, magenta, cyan and black toners is decreased, the sharpness of an image in the main scanning direction drops, which is a problem to be solved. Contrary to this, assume that the optical path length of the laser beam to be irradiated to an image bearing member for forming a toner image using an inconspicuous color such as pale toner or transparent toner is decreased. Even in such a case, when the freshness of the image in the main scanning direction drops, an influence upon the image is small because it is an inconspicuous color.